The present invention relates to an electrical machine having a plurality of permanent magnets which are distributed in the electrical machine in an optimized manner. In particular, temperature profiles in the electrical machine are taken into account when distributing the permanent magnets.
In electric drives for passenger cars, for example hybrid vehicles or battery-operated vehicles, synchronous machines having permanent magnets (PSM: permanent-magnet synchronous machines) are predominantly used as electric motors. The magnets which are used in this case are mainly NdFeB magnets. These motors have a high torque at low rotation speeds and a very high degree of efficiency. Given an optimized arrangement of the magnets, a high power can be achieved over virtually the entire rotation speed range. The mass of the magnets used is up to several kilograms. A further field of application for permanent-magnet synchronous machines is wind generators. The mass of the magnets of a wind generator of this kind can be several 100 kg up to over 1000 kg.
The permanent magnets used in the prior art comprise alloys of metals which are called rare earth metals. Examples of metals of this kind include neodymium and praseodymium. The heavy rare earth metals dysprosium and terbium are also used. The costs of these magnets have risen greatly in recent years on account of the increased demand in the fields of electromobility and regenerative power generation. These magnets are fitted in electrical machines in a manner distributed either in the stator (external rotor) or rotor (internal rotor). NdFeB magnets are often used in the prior art since they have the highest flux density currently known. However, these magnets have the disadvantage that they cannot withstand an opposing magnetic field, which is applied from the outside, at high temperatures and are demagnetized under these conditions. During operation of an electrical machine, the conversion of electrical energy into mechanical energy or vice versa results in heat loss. As a result, the components of the electrical machine are heated. When a critical temperature of the magnets of more than approximately 80° C. is reached, the magnetic field strength of the magnets is successively and irreversibly lost.
In order to be able to operate an electrical machine over as high a limit temperature range as possible, to keep the expenditure on cooling low and to be able to withstand high continuous powers for as long as possible, the NdFeB magnets are alloyed with dysprosium and/or praseodymium. However, the cost of dysprosium and praseodymium is approximately ten times as high as that of neodymium.
The invention is based on the object of providing an electrical machine which firstly has temperature-resistant magnets and secondly reduces costs.
The electrical machine according to an embodiment of the invention comprises at least one first permanent magnet and at least one second permanent magnet. The electrical machine further comprises a first region, the temperature of said first region being in a first temperature range during operation, and a second region, the temperature of said second region being in a second temperature range during operation and the maximum operating temperature of said second region being higher than the maximum operating temperature in the first region. The at least one second permanent magnet has a higher maximum working temperature than the first permanent magnet. The maximum working temperature is below the temperature at which the magnetic field strength of the permanent magnet decreases irreversibly due to temperature and on account of a magnetic field which is applied to the permanent magnet from the outside. The first permanent magnet is arranged in the first region and the second permanent magnet is arranged in the second region.
An electrical machine within the meaning of the present invention can be both an electric motor and a generator.
According to the invention, the inventors of the present invention propose that the choice of materials for the permanent magnets takes into account the temperature distribution in the electrical machine. Therefore, magnets which are subjected to relatively high thermal loads are provided, for example are alloyed, with materials which are more resistant. The magnets which are subjected to relatively low thermal loads are provided, for example alloyed, with a lower proportion of material of relatively high magnetic quality.
The magnetic field strength of a permanent magnet decreases irreversibly depending on the working temperature and a magnetic field, which is applied to the permanent magnet from the outside, when limit values are exceeded, wherein the magnetic field can be generated by a winding (solenoid) of the electrical machine. The magnetic field which is generated by the solenoid can be directed oppositely to the magnetic field of a permanent magnet. In the case of a high field strength, which is applied from the outside, in the permanent magnet, it is necessary to maintain a relatively low working temperature in order to not damage the permanent magnet. The field strength in machines which are severely stressed (torque and/or volume) is generally larger than in the case of machines which are less severely stressed. A 5% field strength is usually always specified for a specific working temperature (HD05). This means that, when the field strength, which is applied from the outside, in the magnet reaches this value, the magnet loses 5% of its remanence flux density. It therefore becomes 5% weaker. The further the external field strength surpasses the field strength HD05, the greater the damage. This damage occurs immediately after the field strength is exceeded once. This is not an ageing effect.
The maximum working temperature can be at least 5° C., preferably at least 10° C., more preferably at least 15° C., extremely preferably at least 20° C., most preferably at least 25° C., below the temperature at which the magnetic field strength of the permanent magnet decreases irreversibly due to temperature. The temperature at which the magnetic field strength of the permanent magnet decreases irreversibly due to temperature can be the temperature at which the macroscopic orientation of the Weiss domains of a permanent magnet is lost on account of a magnetic field which is applied to the at least one permanent magnet from the outside. The magnetic field strength of the permanent magnet can be generated by the remanence flux density. The magnetic field which is applied from the outside can be generated by a stator winding or by other solenoids. The physical principles relating to permanent magnets are known to a person skilled in the art and do not need to be explained further in this document for reasons of conciseness.
The maximum working temperature of the at least one solenoid and a maximum field strength of the magnetic field which is applied to the at least one permanent magnet from the outside can be at least 5%, preferably at least 10%, more preferably at least 20%, below the values at which the magnetic field strength or remanence flux density of the permanent magnet decreases irreversibly due to temperature.
The second permanent magnet can contain a higher proportion of dysprosium, praseodymium, terbium and/or samarium cobalt than the first permanent magnet. Magnets which are subjected to relatively high thermal loads are alloyed with a higher proportion of dysprosium (Dy) and/or terbium (Tb) and/or praseodymium and/or samarium cobalt than the magnets which are subjected to relatively low thermal loads. Therefore, the costs of the magnets and of the electrical machine overall are reduced by using higher-quality and more expensive magnet materials for the individual permanent magnets in a region of the electrical machine which is subjected to relatively high thermal loads and by using lower-quality and therefore more cost-effective materials in a region of the electrical machine which is subjected to relatively low thermal loads.
The first region and/or the second region can be located in the rotor. The second region can be located closer to the axis of the rotor in the radial direction than the first region. This design is based on the knowledge that a higher temperature prevails in the interior of the rotor than in the edge regions of the rotor.
The rotor can be of substantially cylindrical design, that is to say the outer contour of the rotor can be substantially cylindrical. Given this design, the first region can be located closer to the base surface of the cylindrical rotor than the second region. The first region can be located closer to the edge of the rotor than the second region, for example both in the radial direction and in the axial direction.
The at least one first permanent magnet and the at least one second permanent magnet can be arranged at least partially on an electrical sheet.
The electrical machine can be a synchronous machine. The electrical machine can be a generator and/or a motor.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.